Letter 210, point 9 states in part. "There, put in command by Sauron, he is given an added demonic force."

The way I have always read that is that the WKings power as increased due to being in command of Saurons forces as Sauron increases his war effort. As the ringwraiths "...peril is almost entirely due to the unreasoning fear they inspire.." as Saurons power grows through his action and his will concentrates on his war effort, due to the nature of the ringwraiths the fear they inspire also grows. As exhibited at the black gate after his downfall when his will no longer drives his army, even the least of his minions previously driven by his will are bereft of direction. So the only point I was making was that the WKing was no more specifically or singularly increased in type of power or will than most of Saurons servants. Due to the nature of his being any increases were more marked and he would have been much more terrifying to the armies of the west but it should not be thought of that he had any more special powers that would help him in a fight against Gandalf because in the same note Tolkien also says "They have no great physical power against the fearless." That is why Gandalf and Snowmane could face him and that is how Eowyn and Mery were able to face him and defeat him amongst other devices at their disposal (blade of Westernesse) because they ovetcame their fear of him.